Many semiconductor packages utilize a metal lead frame to provide electrical interconnects between an integrated circuit die and external components. Very small electrical wires referred to as “bonding wires” are often used to electrically connect I/O pads on the die (frequently referred to as “bond pad”) to corresponding leads/contacts in the lead frame. Typically, the die, the bonding wires and portions of the lead frame are encapsulated in plastic for protection, while leaving portions of the lead frame exposed to facilitate electrical connection to external devices.
Many lead frames include a die attach pad (DAP) that supports the die during assembly of the package. In some such packages, the die attach pad is exposed on a surface of the package (typically the bottom surface). An exposed die attach pad can help with the thermal management of the package because the die attach pad provides a good thermal conduction path for dissipating excess heat generated by the die. In some exposed DAP packages, the die attach pad is also used as an electrical contact for the package. Most commonly, the die attach pad is used as the ground pad, although in a few packages it may be used as a power pad and theoretically it could alternatively be used as a signal pad.
When the die attach pad is used as an electrical contact, bonding wires are often used to electrically connect one or more ground I/O pads on the die to the die attach pad (a process frequently referred to as “down bonding”). Most commonly, very fine gold or copper wires are used as the bonding wires and the lead frame is formed from copper or a copper based alloy. Since gold does not adhere well to copper, the top surface of the die attach pad (and other relevant portions of the lead frame) are typically plated with a thin film of silver which adheres much better than copper to the gold bonding wires. A problem that occasionally occurs is that the die will sometimes delaminate from the die attach pad during use of the device. Delamination may also occur between the die attach pad and the mold compound that encapsulates the die. When delamination occurs, movement of the die relative to the die attach pad can sometimes detach the down bonding wires from the die attach pad or otherwise break the down bonding wires.
Similar delamination problems can also occur at the leads. For example, delamination sometimes occurs between the molding material and the lead fingers, particularly in the regions of the lead frame that are silver plated. Delamination between the molding material and the leads can also damage the bonding wires.
A representative lead frame suitable for use in a package having an exposed die attach pad is diagrammatically illustrated in FIGS. 1A and 1B. FIG. 1A is a top plan view of the lead frame 100 with a die attached and electrically connected to the lead frame. FIG. 1B is a cross-sectional side view of FIG. 1A taken along section line A-A. The die 102 is wire bonded to the grounded die attach pad 104 using ground bonding wires 106. One end of a ground bonding wire 106 attaches to a ground I/O pad 110 on the die 102, while the other end attaches directly to the die attach pad 104. In designs that use gold ground bonding wires 106, the grounded die attach pad 104 is often plated with silver in order to improve the quality of the bond. In addition, bonding wires 108 connect I/O pads 116 of the die 102 to associated leads 112 of the lead frame 124 in order to electrically connect the die as desired in the integrated circuit design. For example, bonding wires 108 may be used to connect the die 102 to a power source, signal line, or any other suitable electrical connection. The die attach pad 104 is supported by tie bars 118.
Although conventional ground bonding methods work well in many applications, there are continuing efforts to improve the reliability of ground bonds.